1. Field of the Invention
The present invention relates to an electric power controller for a vehicle with stop-start system.
2. Description of the Background Art
In JP8-196006A, there is disclosed a retarder device for use in a general vehicle, while an electric power controller for a vehicle with stop-start system is not disclosed therein. In such the retarder device, a motor/generator is connected to an engine output shaft. A power capacitor for charging and discharging electric power and a vehicle-mounted battery for supplying electric power to an electric load are connected to the motor/generator. A DC/DC converter is connected between the power capacitor and the vehicle-mounted battery. The motor/generator is used as a generator when an engine is braked, and converts a mechanical energy of the engine into an electric energy and charges the power capacitor with the electric energy. Further, the motor/generator is operated as a motor when the engine is started, and directly drives a crank shaft of the engine using the electric energy charged in the power capacitor. The motor/generator is required to generate a high level of torque for directly driving the engine when the engine is started. In the power capacitor disclosed in JP8-196006A, a capacitor having a high voltage of 200V or more is used to meet this requirement.
In the retarder device in JP8-196006A, the DC/DC converter is driven when the engine is stopped to transfer electric power to the vehicle-mounted battery from the power capacitor. The power capacitor has a self-discharging characteristic in the state of being condensed. Accordingly, in order to prevent loss of the electric power energy due to the self-discharging, when the engine is stopped, the electric energy accumulated in the power capacitor is transferred to the vehicle-mounted battery by means of the DC/DC converter. When the engine is stopped, the electric power returns to the power capacitor from the vehicle-mounted battery by means of the DC/DC converter, and when the engine is started, the electric power is fed to the motor/generator from the power capacitor.
As described above, since the retarder device in the related art is configured so that the electric power of the power capacitor is transferred to the vehicle-mounted battery through the DC/DC converter at the time of the engine stop so as to suppress the self-discharging generated when the power capacitor is left unmanaged for a long time, and the electric power transferred to the vehicle-mounted battery is returned to the power capacitor when the engine is started, there is a problem in that the electric energy is decreased before and after the engine stop in consideration of the conversion efficiency of the DC/DC converter. For example, if the conversion efficiency of the DC/DC converter is set to 90%, the total electric energy after the engine starting is decreased to 81% of the electric energy before the engine stop.
Further, even if the retarder device in the related art is modified so that the electric power transferred to the vehicle-mounted battery is not returned to the power capacitor when the engine is started, there is a problem that electric voltage of the power capacitor is decreased in order to transfer the electric energy to the vehicle-mounted battery from the power capacitor when the engine is stopped.